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Waves
Jun 22, 2011 22:46:10 GMT 1
Post by principled on Jun 22, 2011 22:46:10 GMT 1
I recently heard that everything has a wave-like quality but that we are unable to see this in normal objects because their momentum means the wave frequency is so long.. How do we know this? How does this conflict with Newton's first law? P
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Waves
Jun 23, 2011 0:56:32 GMT 1
Post by speakertoanimals on Jun 23, 2011 0:56:32 GMT 1
The frequency goes as the energy, and the wavelength varies inversely with the momentum. So, for a 'large' object like a molecule (or a macromolecule like C60, a buckyball), the wavelength is very, very small.
But it can be measured. As with water waves, and light, you can see diffraction and interference effects with careful experiments.
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Waves
Jun 23, 2011 9:16:35 GMT 1
Post by principled on Jun 23, 2011 9:16:35 GMT 1
Hi STA Thanks for the reply. The example given in the podcast I was listening to was that of a bowling ball which has a wavelike motion with a frequency of millions of km (can't remember the actual figure, nor was amplitude mentioned), although for the observer the ball appears to run straight. This got me thinking about why this would be, bearing in mind that any deviation from a straight line (assuming no gravity etc for the moment) would-according to Newtonian Laws- require the input of a force. I can quite accept this wave motion at sub-atomic level, but it seems counter-intuitive at the macro level. Hence my question about conflict. P
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Waves
Jun 23, 2011 13:19:39 GMT 1
Post by speakertoanimals on Jun 23, 2011 13:19:39 GMT 1
What Newtons laws actually say is that momentum is conserved (that is the continuing in state of uniform motion if no force acts). The same laws of momenum and energy conservation apply on average at the quantum level. The difference is the Heisenberg uncertainty relations, which say that in general we can't know precisely both position AND momentum. Hence if we don't know the momentum precisely, we can't precisely say that momentum conservation has been violated (or not!).
Its akin to the Heisenberg relation between uncertainty in energy, and time. So, virtual particles pairs can pop out of the vacuum (violating energy conservation), provided the energy is paid back quickly enough, and energy conservation holds on average.
The point about the bowling ball example is that there is some small probability that its path isn't a totally straight line, that it jiggles a bit, but it will in general jiggle both ways. On average, its path willl be straight.
To look at it another way, to do an interference experiment, to try and make the bowling ball be in two places at once (like a photon passing through the double slit apparatus), you'd need slits spaced apart according to the de Broglie wavelength. Leaving aside the fact that the bowling ball wouldn't fit through the slits, this tells you why we can't see the being in two places at once wavelike aspect for a bowling ball, the distance is too small. Hence we consider a bowling ball as a definite object with a single definite location.
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